This invention relates to thin metal alloy films which are quasicrystalline and/or approximately quasicrystalline, as opposed to amorphous or crystalline, in nature. It is particularly directed to a method for making very thin films from materials having the general formula AlaCubFecXdIe, where X represents one or more optional alloy elements, I represents manufacturing impurities and a-e are more thoroughly defined herein. It is further directed to applications which take advantage of the unique properties of such films.
Alloy metal films are well known in the art. Alloys having the general formula AlaCubFecXdIe, where X represents one or more elements selected from the group consisting of V, Mo, Ti, Zr, Nb, Cr, Mn, Ru, Rh, Ni, Mg, W, Si and the rare earth elements, and I represents manufacturing impurities, generally present in an amount of less than two percent of the atoms present, are disclosed, for example, by U.S. Pat. No. 5,204,191. U.S. Pat. Nos. 4,595,429 and 4,710,246 disclose a number of aluminum-based alloys characterized by amorphous or microcrystalline structures.
An icosahedral phase was observed in 1984 in a rapidly quenched AlMn alloy. Since that time, many experiments have been carried out to clarify the structure and properties of this new (xe2x80x9cquasicrystallinexe2x80x9d) state of condensed matter. The AlMn alloy is metastabile and many other quasicrystalline phases are also metastable; however, there are ternary (and more complicated) quasicrystalline alloys which have evidenced thermodynamic stability. The AlCuFe alloys, for example, provide stable pure icosahedral phases of high structural quality and peculiar electronic, mechanical and magnetic properties. Notably, such alloys are characterized by unusually high electrical resistivity values, which increase as the structural quality of the sample is improved; a very low thermal conductivity; a low density of states at the Fermi level; a strong composition dependence of the resistivity and the Hall coefficient and a diamagnetic susceptibility; a low surface energy with small coefficient of friction and almost non-stick properties.
The AlCuFe alloys have typically been prepared either by melt spinning or long term annealing of bulk ingots. Monograins of millimeter size have been produced by these techniques. Coatings that are several microns in thickness have been obtained by plasma spraying three elements onto liquid nitrogen cooled substrates and then annealing the resulting deposits. Binary metastable quasicrystals have also been produced by solid state diffusion of either sputtered or evaporated layers. Decagonal Al3Pd phases have been obtained by the lateral diffusion of Al islands on a Pd thin film.
This invention provides an article of manufacture including a quasicrystalline metal alloy film less than about 10,000 xc3x85 thick. While 10,000 xc3x85 is not regarded as a xe2x80x9ccriticalxe2x80x9d boundary, it represents the approximate upper limit of film thickness at which the films of this invention are currently believed to be distinguishable from films of similar composition produced by other means. The film has a composition of the general formula AlaCubFecXdIe, wherein X represents one or more optional alloy elements and I represents manufacturing impurities. In typical films, a=100xe2x88x92bxe2x88x92cxe2x88x92dxe2x88x92e; 24 less than b less than 26; 12 less than c less than 13; 0 less than d less than 10; and 0 less than e less than 3. More preferred film compositions are approximately: a=100xe2x88x92bxe2x88x92cxe2x88x92dxe2x88x92e; 24.4 less than b less than 26.0; 12.0 less than c less than 13.0; d=0; e=trivial. The film is formed by depositing, in sequence on a substrate, by radio frequency sputtering, a stoichiometric amount of each respective constituent material and then annealing the layers, and hereby forming the film through solid state diffusion. The substrate may comprise an electrical insulator or a conductor, such as an alloy, and the film may be utilized as a component of an electronic device or as the casting of a mechanical device. The substrate may, alternatively, comprise a wear surface or cutting edge, and the film may be utilized as a protective coating and/or for thermal isolation and/or enhanced performance.
According to the invention, very thin films, typically less than about 10,000 xc3x85, ideally from about 100 xc3x85 to 10,000 xc3x85 thick, are produced by a technique involving depositing xe2x80x9cstoichiometricxe2x80x9d ratios of selected alloy elements in sequence on an oscillating substrate, and then annealing the deposited layers to produce an alloy having a composition within a quasicrystalline phase. The term xe2x80x9cstoichiometricxe2x80x9d is used in this disclosure to indicate the presence of atoms of selected elements in a predetermined numerical ratio. Accordingly, layers of the selected alloy elements are deposited in sequence in precise amounts on a suitable substrate. This process essentially allows an atom by atom deposition with control on the amount of each element. Within practical limits, annealing of those layers will produce an alloy composition reflecting the stoichiometric ratio of the layers. This approach has been found effective to produce very thin films possessing excellent properties for a variety of practical applications. The films of this invention are ideally comprised of ternary stable AlCuFe quasicrystals prepared by the solid state diffusion of Al, Cu and Fe layers, an exemplary such film being formed from layers of Al, Cu and Fe deposited in sequence in thickness ratios of approximately 7.0/1.0/2.0, respectively, prior to annealing.
While this disclosure emphasizes AlCuFe alloys, the film formation techniques of this invention are regarded as being generally applicable to any alloy composition within a quasicrystalline region of a phase diagram descriptive of any alloy system of interest. The aluminum-rich alloys described by U.S. Pat. Nos. 4,595,429, 4,710,246 and 5,204,191 are of primary interest because of the interesting properties observed to be characteristic in these alloys.
Practical applications for films produced by this invention include electronic circuits for devices; coatings to reduce friction and wear in machines including monostructures, bearings, cutting tools and circuits; protection of hard disk surfaces against hard crashes and providing low friction surfaces to devices such as magnetic storage drives and magnetic tape or disc heads. The high hardness and low friction characteristics of quasicrystalline materials are well suited for coatings of cutting surfaces, such as razor blades, surgical blades, and knives, generally offering smoother performance with less tear of flesh or other forms of degradation typically encountered in cutting various materials. Such coatings impart non-stick characteristics to the blades in use, and extend their useful lives. Electrosurgical blades coated with a thin film of quasicrystalline AlCuFe material offer superior mechanical and electrical performance, for example, compared to presently-used TEFLON-coated blades or non-coated blades. Similar coatings may also be applied to the critical surfaces of sports equipment, notably skates and skis.
The low thermal conductivity of the films of this invention enables them to function well in a variety of applications. For example, a quasicrystalline coating may function as a heat shield or to maintain a thermal gradient.
Various substrates are practical. Strontium titanate, silicon, silicon dioxide, sapphire and steel are exemplary substrates. Very thin films can be fabricated in accordance with this invention, and both the thickness and purity level of the film can be well controlled. The process may be conducted so that film formation takes place by solid state diffusion, without going through a liquid phase. Lower anneal temperatures (600xc2x0 C. for a particular alloy compared to the 800xc2x0 C. temperature required for the same alloy in bulk ingot form) are effective. A two-step heat treatment is presently preferred. In the specific case of AlCuFe, the first step is conducted at a relatively lower temperature to form the first-to-form aluminum-rich alloy; i.e. Al3Fe. The second step is conducted at a relatively higher temperature to promote formation of the quasicrystalline phase. Films of certain alloys consist of grains smaller then 0.5 xcexcm in diameter.
A notable benefit of the formation technique of this invention; i.e., the combination of sputtering and solid state diffusion, is the resulting excellent adherence of the quasicrystalline film to the substrate.
According to the invention, various articles of manufacture, such as electrosurgical scalpels, razor blades, and electronic components, comprise a quasicrystalline AlCuFe alloy film formed by depositing in sequence on a substrate through radio frequency sputtering a stoichiometric amount of each respective alloy material, and then annealing those layers to form the film through solid state diffusion. The film is ideally comprised of ternary stable AlCuFe quasicrystals prepared by the solid state diffusion of Al, Cu, and Fe layers. Layers of Al, Fe and Cu are most preferably deposited in sequence in thickness ratios of 7.0/1.0/2.0, respectively, prior to annealing. Certain articles produced by this invention utilize the film for its thermal isolation properties. Other articles utilize the film to maintain a temperature gradient. In other articles, the substrate may be associated with the cutting edge of a knife, scalpel, razor blade or electrosurgical blade. In still other articles, the substrate is an electrical insulator or conductor, and the film is utilized as a component of an electronic device. Other articles utilize the film as a coating for wear surfaces. In most instances, the film will be less than about 10,000 xc3x85 thick, typically, less than about 1000 xc3x85 thick.